Single-phase or three-phase power converters which convert direct voltage from the source into grid-compatible alternating voltage, for a solar or photovoltaic generator with a high output voltage exist. An intermediate circuit capacitor is conventionally connected downstream from the generator and is followed by the power converter. A sine-wave filter is connected at the output of the power converter. The energy is fed into the grid via a transformer.
When photovoltaic systems are started up, the high open-circuit voltage of the generator is problematic. In open-circuit operation, the solar generator has a high output voltage. The open-circuit voltage is generally more than 20% higher than at the operating point during feeding, what is known as the MPP (maximum power point) voltage. In the MPP tracking process, a generator operating point of optimal power production is set. The elevated generator voltage during open-circuit operation must not cause any damage to the electronic systems during the start-up process.
Additional external circuitry, such as a resistance chopper, is also typically used. The resistance chopper is connected in parallel with the DC intermediate circuit or parallel with the power converter input and reduces the intermediate circuit voltage during the start-up phase. The drawbacks of these devices are the high losses experienced during the start-up phase and the high load on the resistance chopper.
In another solution described in DE 20 2006 001 063 U1, a step-down converter is connected upstream of the three-phase power converter. In this case, the step-down converter reduces the photovoltaic generator voltage to a permissible value if necessary. The drawback is that when the generator voltage is low, the step-down converter switch must be permanently closed, thus causing considerable forward power losses. In addition, the switch and diode of the step-down converter must be configured for the maximum generator voltage, i.e. the open-circuit voltage. This thus requires components of greater electric strength than required in operation at the MPP operating point. These components are more expensive and exhibit higher power loss. According to DE 20 2006 001 063 U1, the switches of the three-phase power converter are clocked at mains frequency, which is a drawback since sinusoidal current is not supplied.